1. Field of the Invention
The present invention relates to an ultrasonic image processor and an ultrasonic diagnostic instrument which provide information effective for medical diagnosis by estimating a velocity of a biological tissue such as cardiac muscle, etc., processing estimated velocity information, and outputting local motion information of the tissue.
2. Description of the Related Art
In general, it is very important for diagnosis of biological tissues such as cardiac muscles, etc. to objectively and quantitatively estimate functions of the tissues. Various quantitative estimation methods using a heart as a sample have been tried in a field of imaging diagnoses using ultrasonic image processors. A representative example thereof is a tissue tracking imaging (TTI) method.
In the tissue tracking imaging method, for example, as disclosed in Japanese Patent Application No. 2002-272845, parameters of local displacements and distortions obtained by integrating signals derived from velocity information while tracking positions of tissues with motion are imaged as motion information of the tissues. According to this method, images of distortions and displacements of local cardiac muscles of a heart could be formed and displayed using, for example, minor axis images, and the analysis of temporal variation of image output values at local areas is supported. In the case of using the minor axis image, an important analysis target of a heart is a thickening (variation in thickness), but in the tissue tracking imaging method, components associated with the thickening are detected and imaged through an angle correction, so that a concept or setting of a motion field toward a contraction center (contraction motion field) is utilized. In the tissue tracking imaging method, the position of the contraction center is temporally moved in consideration of influence of the translation motion (also referred to as “translation”) of the whole heart and the method is applicable to the temporally-variable motion field. Therefore, the variation of the contraction center position due to the translation motion can be coped with.
However, the tissue tracking imaging method has a room for improvement as described below.
First, in the tissue tracking imaging method, for the purpose of simplification in processing, the detection of velocity components that are analysis targets is an approximation. That is, the detected velocity of a predetermined position includes an error corresponding to a velocity component of movement (that is, a translation velocity component) of a contraction center due to movement of a body. This situation will be described with reference to FIGS. 1A and 1B.
FIG. 1A is a schematic diagram of a minor axis image of a heart illustrating a conventional angle correction with no movement of a contraction center. FIG. 1B is a schematic diagram of the minor axis image of a heart illustrating a conventional tracking with movement of a contraction center. As shown in FIG. 1A, when the velocity component of movement of the contraction center is small to be neglected, there is no problem even if the estimation of Pt or Vp:contt (where t is a time phase) is performed while neglecting the velocity component of movement from the viewpoint of Vp:contt=Vp:obst/cosθ.
However, as shown in FIG. 1B, when a translation motion of the whole heart exists substantially, the movement velocity component of the contraction center is added to Pt, and strictly considering, the movement velocity component Vc:transt of the contraction center is added to Vp:obst and thus is observed. Conventionally, since this influence is neglected, a large movement of the contraction center causes an error.
Second, there is known that the movement of the whole heart is substantially very complex and a twist motion in which the whole left ventricle is contracted to efficiently send out blood exists in systole. In the minor axis image, for example, as disclosed in “Myocardial Velocity Gradient Imaging by Phase Contrast MRI With Application to Regional Function in Myocardial Ischemia” A. E. Arai et al, Magnetic Resonance in Medicine 42: 98-109, 1999, the twist motion is detected as a rotation component and studies thereof have been advanced recently using an MRI. Therefore, when it is tried to detect only the thickening component with more accuracy, the rotation component should be considered as well as the translation component.
Conventionally, from this point of view, various methods of removing the whole movement of a heart have been studied. For example, as disclosed in “Wall Motion Imaging using Tissue Doppler method”, Mine et al, Hikosy's collection of works on medical theories 63: P671-672, 1993, there is a technique of obtaining only the thickening component by modeling the movement of a heart and estimating and removing the translation component which is considered as the most surest through the method of least squares (through repeated calculation using correlation coefficients) using velocities of plural points. According to this technique, it could be expected to adaptively estimate the rotation component, but there is a serious problem in that the movement of a heart is modeled. Specifically, since large variation of velocity distribution in a cardiac muscle deviates from the model (a hypothesis that a cardiac muscle is constantly contracted and expanded), it causes an error. It is known that the velocity distribution is not constant inside the cardiac muscle. Finally, from the viewpoint of trying to analyze the local distribution of motion information inside the cardiac muscle, the above technique could not avoid contradiction.
Third, in the tissue tracking imaging method, a limit area for imaging exists and a long data processing time might be required. That is, in the tissue tracking imaging method, the velocity component of a predetermined position of a cardiac muscle is obtained using a technique based on the tissue Doppler method or a technique based on two-dimensional velocity detection (pattern matching of a received ultrasonic RF signal and a B mode signal, etc.). However, in the former, a limit of a Doppler angle exists partially, so that there exists an area of which the velocity cannot be detected in principle. On the other hand, in the latter, the limit of a Doppler angle does not exist, but a very large calculation time is required for calculating a characteristic amount as an image, so that there is a difficulty in spread for clinical application.